volatile provides absolutely no guarantee thread-safety in SMP
environments.  volatile was designed to access memory locations
in peripheral hardware directly; not for SMP.  If volatile is
needed to work properly on SMP, then it is only hiding subtle
bugs.

volatile only prevents the /compiler/ from making optimizations
when accessing variables.  CPUs do their own optimizations at
runtime so it cannot guarantee registers of CPUs are flushed
to memory cache-coherent access on different CPUs.

Furthermore, the thread-communication via condition variables
between threads sharing audio formats already results in memory
barriers.